I'm getting an error using Tequila API (error 403). How can I remedy this?

I've been trying to get Tequila API working for a project, and I'm running into a 403 error.

requests.exceptions.HTTPError: 403 Client Error: Forbidden for url: https://api.tequila.kiwi.com/v2/flights_multi

I've checked my API key, it is correct.

I've tried using the API key as a parameter instead (end up getting error 400), I've tried removing the other two header parameters (Content-Encoding and Content-Type), and I've looked over the "requests" parameter for any errors. I've also tried using both json and params in the post request.

Here is the tequila API code documentation I've been using:General Info: https://tequila.kiwi.com/portal/docs/user_guides/search_api__general_information_Multicity API: https://tequila.kiwi.com/portal/docs/tequila_api/multicity_api

Here is my code:

import requestsfrom datetime import datetime, timedeltafrom monthdelta import monthdeltaimport os# Flight Search endpoint and API key.FLIGHT_SEARCH_ENDPOINT = "https://api.tequila.kiwi.com/v2/flights_multi"FLIGHT_SEARCH_API_KEY = os.environ.get("FLIGHT_SEARCH_API_KEY", "Please provide a FLIGHT_SEARCH_API_KEY.")# City to fly from. We'll use London for this example.FLY_FROM = "LON"# Constants for dates tomorrow and six months from tomorrow (our flight date range).TOMORROW = str((datetime.now() + timedelta(days=1)).strftime("%d/%m/%Y"))SIX_MONTHS = str((datetime.now() + timedelta(days=1) + monthdelta(6)).strftime("%d/%m/%Y"))# List of IATA codes for cities to get flight info.# In order: Paris, Berlin, Tokyo, Sydney, Istanbul, Kuala Lumpur, New York City, San Francisco, Cape Town.iata_codes = ["PAR", "BER", "TYO", "SYD", "IST", "KUL", "NYC", "SFO", "CPT"]# This class is responsible for talking to the Flight Search API.class FlightSearch:    def __init__(self):        self.header = {"Content-Type": "application/json","apikey": FLIGHT_SEARCH_API_KEY,"Content-Encoding": "gzip"        }        request_list = []        for code in iata_codes:            request_params = {"fly_to": code,"fly_from": FLY_FROM,"date_from": TOMORROW,"date_to": SIX_MONTHS,"adults": 1            }            request_list.append(request_params)        self.params = {"requests": request_list}        self.response = requests.post(url=FLIGHT_SEARCH_ENDPOINT, headers=self.header, json=self.params)        self.response.raise_for_status()        print(self.response.text)flight_search = FlightSearch()

I've written above what I tried. I'm expecting a 200 return, to get the required data. Thanks for the help!